Overview: Transmission and Prevention

HIV is transmitted by certain specific behaviors involving the exchange of bodily fluids between people. Unlike many other diseases, HIV is not transmitted through routine casual contact, through the air, or through insect bites.

Sexual activities can transmit HIV when they involve direct contact between the HIV-infected bodily fluids (such as semen, vaginal secretions, and blood) of one person and the mucous membranes (such as in the vagina, rectum, and mouth) of another. This transmission can be reduced by avoiding these behaviors or by proper use of barriers such as condoms and dental dams.

Needles used to inject drugs can transmit HIV when they are used by more than one person. Needles should never be shared, but if they are shared they should be thoroughly cleaned between uses.

HIV can be transmitted from person to person through the transfusion of blood and blood products. However, routine screening for HIV of the blood supply has dramatically reduced this mode of transmission, at least in the developed world.

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HIV can be transmitted from mother to fetus/baby while it is still in the uterus, during the delivery process, and through breast-feeding. There is strong evidence that use of antiviral medications during pregnancy can reduce maternal transmission of HIV.

People become infected with HIV due to behaviors, not because of membership in any particular "high risk group," making HIV infection a possibility for almost everyone but an inevitability for almost no one.

Coverage in the Encyclopedia

The entries in this section emphasize behaviors rather than risk groups. Many of these entries have companion entries for specific types of individuals (e.g., Gay Men; Hemophilia, People with; Injecting Drug Users; etc.), all of which are listed in the section below on "Impacted Populations."

The entries on sexual transmission and prevention cover the specific sexual behaviors and the main bodily fluids that are most frequently responsible for HIV transmission. Other entries cover sexual acts that rarely, if ever, cause HIV transmission but that cause concern among many people. The entries on sexual transmission also cover the social behaviors and attitudes within which sexual transmission of HIV occurs. In addition, these entries also include coverage of the most important approaches used to prevent the sexual transmission of HIV.

The entries on needle sharing among injecting drug users (IDUs) discuss the various mechanisms of IDU transmission. They also cover related topics concerning the social and policy contexts within which drug use occurs and how injecting drug use can be made less risky.

Other entries cover maternal transmission, transmission through blood and blood products, and accidental exposures. Several important general prevention strategies are also covered.

The Intersection of Host and Virus: Risk Factors for HIV Infection

The primary mission of HIV research is simple: to determine how HIV is spread, causes disease, and can be treated in human beings. Although it is true that many advances have been made in each of these particular areas of research, it is also true that research can never yield 100 percent accurate results and there will always be exceptions to every rule. For example, research has yet to determine why some people infected with HIV progress faster than others. Likewise, research has yet to yield a vaccine or cure that will be effective in everybody. The same goes for HIV transmission: some people are more prone to becoming infected with the virus than others.

Despite the fact that researchers have been able to isolate HIV in virtually every fluid secreted by the body -- including tears, saliva, urine, blood, semen, mucus, and vaginal secretions -- HIV still remains, relatively speaking, a virus that is not easily transmitted from one person to another. HIV's ability to successfully leave one person's body and enter another's is highly dependent on secondary factors. These secondary factors -- called risk factors -- are based on both biological and behavioral characteristics.

The biological factors associated with HIV transmission are numerous and complicated. Characteristics on which a determination of the probability of sexual transmission depends include the concentration of HIV in the fluid, the integrity or relative vulnerability of involved mucous membranes (such as those inside the anus, vagina, or mouth), the duration of exposure, and the strain of virus transmitted. For example, HIV can be found in saliva, tears, blood, breast milk, semen, urine, and vaginal and anal secretions. The level of HIV in each of these fluids varies greatly, however. HIV levels are high in blood, semen, and breast milk; moderate in anal and vaginal secretions (although higher for the latter during menstrual periods); and low in saliva, tears, and urine. Thus, the type of bodily fluid someone comes into contact with is in itself a significant biological characteristic.

Of course, HIV is not transmitted simply by coming into contact with an HIV-infected fluid; the fluid must enter another person's body to transmit HIV. The way in which the fluid enters the body is commonly referred to as the route of transmission and is yet another important biological characteristic that may be associated with HIV transmission. HIV-infected bodily fluids can be swallowed, accepted into the vagina or rectum, or injected directly into the bloodstream. HIV is much more likely to directly enter the body and cause infection through some routes more than others, however.

Based on what is known about these biological characteristics, epidemiologists have been able to devise calculations to determine the level of risk, or likelihood, of becoming infected with HIV. By considering both the type of bodily fluid and the route of potential transmission, it is possible to assess someone's general level of risk of being infected with HIV. Levels of risk have been assessed for every possible form of human contact. For example, shaking hands or hugging is not associated with any sort of risk. On the other hand, injecting drug users who share needles bear a very high risk of HIV transmission, given that HIV-infected blood often remains in or on the hypodermic needle and can easily be injected directly into another person's bloodstream.

Each type of sexual contact has its own level of risk, and many people's sexual lives are structured so that some risk of HIV infection exists. Yet, the definitive level of risk associated with each and every sex act cannot always be calculated. For example, anal sex -- in which HIV-infected semen comes into contact with tiny abrasions of the anal wall -- is highly risky for HIV transmission. Oral sex -- in which semen or vaginal fluids are taken into the mouth and/or swallowed -- is considered a less likely route of HIV transmission. A single, penetrative sexual contact with an infected partner does not always lead to infection, because HIV infection occurs only if biological characteristics are suitable. Repeated exposures increase the odds for creating suitable conditions for infection.

Behavioral factors are directly related to biological factors. Because many of the latter do not occur inadvertently but can be avoided by way of behavioral change, behavior plays a large role in the continual spread of HIV. Examples of behavioral characteristics include whether or not individuals practice abstinence, use condoms, abuse drugs or alcohol, and use clean hypodermic needles.

When behavioral characteristics are calculated along with biological characteristics, the risk-factor equation becomes more complete. For example, someone who uses a condom every time he or she engages in anal or vaginal sex is much less likely to become infected with HIV than someone engaging in similar sex acts without a condom. Likewise, an injecting drug user is much less likely to become infected with HIV if he or she cleans a previously used hypodermic needle before using it to inject drugs again.

There are, however, cases in which both biological and behavioral characteristics are less definitive. Oral sex, for example, is generally associated with a relatively lower biological risk of HIV transmission. Many HIV-prevention experts argue that condom use during oral sex is still necessary, despite the low risk. Yet, many are reluctant to practice safer oral sex with a condom.

Unlike biological risk factors, which are determined based on anatomic and physiological knowledge and for which specific HIV-prevention measures can be devised, behavioral risk factors are often the most difficult to calculate and control. Despite the knowledge that condom use can prevent HIV transmission, many men and women continue to have unprotected sex. Perhaps this reality reflects the greatest shortcoming of scientific research: that, in addition to the fact that no scientific principle will prove true for everyone, no one type of behavior-modification approach will be effective in preventing the spread of HIV.

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